Rewrite FileSearch and improve ScanDirectoryTree.

- FileSearch is now just one function, and it converts the original glob
  into a regex on all platforms rather than relying on native Windows
  pattern matching on there and a complete hack elsewhere.  It now
  supports recursion out of the box rather than manually expanding
  into a full list of directories in multiple call sites.

  - This adds a GCC >= 4.9 dependency due to older versions having
  outright broken <regex>.  MSVC is fine with it.

- ScanDirectoryTree returns the parent entry rather than filling parts
  of it in via reference.  The count is now stored in the entry like it
  was for subdirectories.

- .glsl file search is now done with DoFileSearch.

- IOCTLV_READ_DIR now uses ScanDirectoryTree directly and sorts the
  results after replacements for better determinism.

Source/Core/DolphinWX/Config/InterfaceConfigPane.cpp

@@ -162,12 +162,10 @@ void InterfaceConfigPane::LoadGUIValues()
 
 void InterfaceConfigPane::LoadThemes()
 {
-	CFileSearch::XStringVector theme_dirs;
-	theme_dirs.push_back(File::GetUserPath(D_THEMES_IDX));
-	theme_dirs.push_back(File::GetSysDirectory() + THEMES_DIR);
-
-	CFileSearch cfs(CFileSearch::XStringVector(1, "*"), theme_dirs);
-	auto const& sv = cfs.GetFileNames();
+	auto sv = DoFileSearch({"*"}, {
+		File::GetUserPath(D_THEMES_IDX),
+		File::GetSysDirectory() + THEMES_DIR
+	}, /*recursive*/ false);
 	for (const std::string& filename : sv)
 	{
 		std::string name, ext;